1. Field of the Invention
The present invention relates to a developing device for an electrophotographic printer, and more particularly to a developing device of a multi-stage roll magnetic brush type in a center feed system.
2. Description of the Related Art
A multi-stage roll developing system can provide a higher developing capability than a single roll developing system. Because of such an advantage, the multi-stage roll developing system can deal with a wide area image printing and improve image quality. In addition, the multi-stage developing system can reduce the toner content of a developer and lower the rotating speed of a developing roll. Thus, the spraying of toner and the load to the developer will be decreased so that the long life of the developer can be assured.
The multi-stage roll developing system can be classified roughly into two systems of a uni-directional feed system, as described in Japanese Patent Publication No. 53-41534 (1978) in which plural (two to four) developing rolls are rotated in the same direction and a center feed system in which two developing rolls are rotated in opposite directions. The identical direction (or sense) rotation development in the unidirectional feed system in which the rotation direction of a developing roll is identical to that of a photosensitive body can provide a high developing capability, but has disadvantages that it gives background fog, missing of the front end of an image and noticeable traces of a magnetic brush.
The opposite direction rotation development in which the rotation direction of the developing roll is opposite to that of the photosensitive roll can provide stabilized image quality because it gives missing of the rear end of the image, however less background fog and less noticeable traces of the magnetic brush. However, the opposite direction rotation development, in which the relative speed of the photosensitive body and the developing roll is low and the effective toner amount in contact with the above photosensitive body is little, provides reduced developing capability. On the other hand, the center feed system, which has both developing rolls for the identical rotation development and opposite direction rotation development, respectively, can obviate defects of above two developing systems.
Referring to FIG. 2, an explanation will be given of an example of the developing device in the center feed system, as disclosed in Japanese Patent Examined Publication No. Sho 62-45552 (1987).
Two developing rolls 5 and 14 are arranged oppositely and adjacently to a photoconductive photosensitive body 1. Each of the developing rolls 5 and 14 is composed of an inner magnet and an outer sleeve. The developing roll 5 serves for the opposite direction rotation development whereas the developing roll 14 serves for the identical direction rotation development. A developing agent or developer 3 in a developer container 2 is carried below the developing roll 5 by a carrying member 4. The developer 3 is deposited on the surface of a sleeve 11 of the roll 5 owing to the magnetic aspiration or induction force of a fixed magnet 8 of the roll 5. The deposited developer 3 moves along the rotation direction of the sleeve 11 of the roll 5. The amount of the developer 3 to be deposited for the opposite direction rotation development is secured by a distributing member 21. The developer 3 is carried to a development area and subjected to the opposite rotation development. Thereafter, the developer 3 is returned to the developer container 2. The rest of the developer 3 removed by the distributing member 21 is deposited on the surface of a sleeve 16 of the developing roll 14 owing to the induction force of a fixed magnet 15 of the roll 14. The developer 3 deposited on the roll 14 moves along the rotation direction of the sleeve 16 of the roll 14. The amount of the developer to be deposited for the identical direction rotation development is also secured by the distributing member 21. The developer 3 is subjected to the same direction rotation development. Thereafter, the developer 3 is returned to the developer container 2. Incidentally, reference numeral 23 denotes a toner hopper and 24 denotes a toner feed roll.
The above prior art center feed system of the developing device, in which a single opposite direction rotation roll and a single opposite direction rotation roll are provided, can provide sufficient developing capability in a low or middle speed printing, but cannot deal with high speed printing used for a wide area image. Referring to FIG. 3, an explanation will be given of an example of the developing device in the center feed system using three or more developing rolls, as disclosed in Japanese Patent Examined Publication No. Hei 1-45552 (1989).
Three developing rolls 5, 6 and 14 are arranged oppositely and adjacently to a photoconductive photosensitive body 1. Each of the developing rolls 5, 6 and 14 is composed of an inner magnet and an outer sleeve. The developing rolls 5 and 6, which are located upstream of the roll 14, rotate in an opposite direction to that of the photoconductive body 1 and serves for the first and second opposite direction rotation development whereas the developing roll 14, which is located downstream of the roll 6, rotates in the same direction as that of the photoconductive body 1 and serves for the identical direction rotation development. A developing agent or developer 3 in a developer container 2 is carried below the developing roll 5 by a carrying member 4. The developer 3 is deposited on the surface of a sleeve 11 of the roll 5 owing to the magnetic aspiration or induction force of a fixed magnet 8 of the roll 5. The deposited developer 3 moves along the rotation direction of the sleeve 11 of the roll 5. The amount of the developer to be deposited for the first opposite direction rotation development is secured by a distributing member 21. The developer 3 is carried to a development area where the photosensitive body 1 and the roll 4 are adjacent to each other and subjected to the opposite rotation development in the development area moving in the same direction as that of the photosensitive body 1. Further, the rest of the developer 3 is deposited on the surface of a sleeve 12 of the roll 6 owing to the magnetic aspiration or induction force of a fixed magnet 9 of the roll 6. The deposited developer 3 moves along the rotation direction of the sleeve 12 of the roll 6 and is subjected to the second opposite direction rotation development.
The remaining portion of the developer 3 is carried to the most downstream roll 14. The developer 3 is deposited on the surface of a sleeve 16 of the developing roll 14 owing to the induction force of a fixed magnet 15 of the roll 14. The developer 3 deposited on the roll 14 moves along the rotation direction of the sleeve 16 of the roll 14 in the same direction as that of the photosensitive body 1. The amount of the developer to be deposited for the same direction rotation development is secured by a distributing/control member 25. The developer 3 is subjected to the identical direction rotation development in the same manner as the above opposite rotation direction development.
Most of the developer 3 controlled and scraped off by the control member 25 is returned to the developer container 2. The developer 3, after subjected to the same direction rotation development, joins the developer supplied from the side of the roll 6, and repeats to rotate on the roll 14 until it is scraped off by the controller 25 so as to be returned to the developer container 2.
The prior art system described above can exhibit high developing capability in the high speed printing. However, this system has also the following defect. Since the roll 14 for the identical rotation direction development is arranged at the outlet of the development, the printing quality may be affected by missing of the front edge of an image and traces of a brush which are features of the identical rotation direction developing system. As a result, the printing quality is not stable.
As described previously, in the multi-roll system, the identical direction rotation development in the uni-directional feed system can provide a high developing capability, but has disadvantages that it gives background fog due to excessive application of toner and missing of the front edge of an image. On the other hand, the opposite direction rotation development can provide stabilized image quality with less disorder in the front edge of an image and less background fog than in the identical rotation direction development. However, the opposite direction rotation development, in which the effective toner amount in contact with the above photosensitive body is little, provides reduced developing capability.
As described above, the center feed system having both development manners can obviate the above defects. However, the two roll system in which a single roll is provided for each of the opposite rotation direction development and the identical rotation direction development, as disclosed in Japanese Patent Examined Publication No. Sho 62-45552, cannot sufficiently deal with the wide area printing at a high speed.
For this reason, the developing device in the center feed system using three or more developing rolls, as disclosed in Japanese Patent Examined Publication No. Hei 1-503811, has been required. Such a developing device, however, also has a disadvantage in that it is restricted by the arrangement of the opposite rotation developing roll and identical direction rotation developing roll so that the carrying path of a developer becomes complicated.